Estimate a due date using the first day of your last menstrual period and usual cycle length, or a known conception date. This is a planning estimate; your maternity team may revise it.
Pregnancy due date calculator
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How this estimate works
For a last-period date, the calculator adds 280 days and adjusts by the difference between your cycle length and 28 days. For a known conception date, it adds 266 days. For example, a last period beginning January 1, 2026 with a 28-day cycle gives an estimated due date of October 8, 2026.
Choose the conception option only when you have a reliable conception date. The date of intercourse is not necessarily the date of conception. Irregular cycles and uncertain dates make calendar estimates less reliable.
A due date is not a prediction of the exact birth date
The NHS explains that pregnancy commonly lasts between 37 and 42 weeks from the first day of the last period. A dating scan can provide a more accurate estimate. This calculator does not assess fetal development, pregnancy complications or whether a pregnancy is overdue.
If you conceived through fertility treatment, use the date supplied by your fertility team. For symptoms or concerns about your pregnancy, contact your maternity team rather than relying on a calculator.
